Hybrid vehicles may have first and second prime movers, such as an internal combustion engine, and an electric machine. The vehicle may operate using either prime mover, or using both simultaneously. The vehicle may operate electrically depending on the amount of available electrical power and the vehicle loads, including driver demand and other vehicle accessory loads. When the battery state of charge reaches a lower limit, the engine may be started to provide additional power to the vehicle. The engine may also be started when the power or torque requirement of the vehicle will exceed that available from the electric machine.
When an additional external load on the vehicle is present, such as the vehicle traveling over a grade, the added load on the vehicle caused may exceed the electrical capability of the electric machine, and require the engine to be operating. A delay associated with starting or restarting the engine, or pulling up the engine, during vehicle operation may cause driveline disturbances, or may not meet vehicle drivability expectations.